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well, at least in the cross platform sense. developers should start separating the front and back end in their apps. That way they can make the backend very platform independent and portable and then make the frontend with the native toolkit for each platform.
Gtk and Qt works well in an x11-enviroment but much less so on other platforms suchs as win32 or macos. Even though an app built with these toolkits may be functional on, say macos x, it almost always feels clunky and like a foreign citizen in the host os.
This way, apps will conform to the HIG of the respective OS and generally will feel more enjoyable to work with. You might not get a consistent look across different OS:s but as long as the general layout and menus are the same, I don't think that matters.
I must add that Adobe has succeded pretty well with a cross platform look and feel, though.